Extend ignored-return-value warning to more functions (bug#61730)

Warn when the return value of certain functions is unused.  Previously
this was only done for side-effect-free functions, and for `mapcar`.

These are functions where the return value is important for correct
usage or where ignoring it is likely to indicate a mistake.  The exact
set of functions is tentative and will be modified as we gain a better
understanding of which ones to include.

The current set comprises higher order functions such as `mapcar`
which are not primarily called for the effects of their function
arguments, and list-mutating functions like `nreverse` whose return
value is essential.

* lisp/emacs-lisp/bytecomp.el (byte-compile-form): Add list of
functions to warn about when their value is ignored.
* etc/NEWS: Announce.

1 files changed, 15 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index d20d9f65ac9..c61a9ec3c5f 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -443,6 +443,21 @@ simplified away.
 This warning can be suppressed using 'with-suppressed-warnings' with
 the warning name 'suspicious'.
 
+---
+*** Warn about more ignored function return values.
+The compiler now warns when the return value from certain functions is
+ignored.  Example:
+
+    (progn (nreverse my-list) my-list)
+
+will elicit a warning because it is usually pointless to call
+'nreverse' on a list without using the returned value.  To silence the
+warning, make use of the value in some way, such as assigning it to a
+variable.  You can also wrap the function call in '(ignore ...)'.
+
+This warning can be suppressed using 'with-suppressed-warnings' with
+the warning name 'ignored-return-value'.
+
 +++
 ** New function 'file-user-uid'.
 This function is like 'user-uid', but is aware of file name handlers,